1. Field of the Invention
The present invention relates to heat exchange systems, and particularly to various embodiments of a heat exchanger for photovoltaic panels. The heat exchanger optimizes the flow paths of the coolant therethrough to provide more uniform temperature distribution.
2. Description of the Related Art
Solar energy can be used in a variety of ways to generate electricity. One way is to use solar thermal power systems that utilize solar energy to raise the temperature of a working fluid, which is then used to run a turbine engine. A second way is to use photovoltaic (PV) systems that generate electricity directly from sunlight. The use of PV systems has increased greatly in recent decades due to significant reductions in PV system costs and improvements in PV system component performance. At present, PV modules convert around 12-20% of solar energy into electricity, while the remaining energy increases the panel temperature. This increase in temperature causes the module efficiency to decrease and results in thermal stress development in the module. Cooling of PV panels provides a method to reduce the PV cell temperature to ensure that the PV cells operate at high efficiencies and do not develop structural defects due to high thermal stresses and stress cycling during day-to-day operation. The attached heat exchanger also serves the additional purpose of collecting thermal energy in the form of hot water, which increases the overall energy collection efficiency of the system.
A number of studies have been done to combine the photovoltaic panel with flat plate solar thermal collector into a single collector called the photovoltaic/thermal (PV/T) collector. These include studies in which off-the-shelf PV panels were cooled by heat exchangers, and studies in which custom-made PV/T collectors specially designed for thermal energy collection along with electricity generation were developed. All of these studies used parallel channel heat exchangers. Some commercial auxiliary thermal collectors for off-the-shelf PV panels, as well as custom PV/T collectors, have also appeared in the market.
It has been observed that in parallel channel heat exchangers, flow is not uniformly distributed between the channels. Moreover, PV panels are usually mounted in a sloped configuration, which further increases the non-uniformity of flow. This results in large temperature gradients across the PV modules that are being cooled. This can lead to degradation of electrical performance due to cell-mismatch losses. It has been found that the electrical performance increased by as much as 9% for the case when the flow was most uniform.
A variety of factors control the flow distribution in heat exchangers. These include the ratio of the cross-sectional area of the header to the cross-sectional area of the channels, the arrangement of flow channels that control the flow direction, the fluid inlet conditions, flow resistance inside the header and channels, entrance effects at the openings of the channels, and gravity effects. It has been found that buoyancy effects increase with a decrease in the flow rate through the collector. Several geometric strategies for header design with an objective to achieve uniform flows in all channels of the heat exchanger have been developed as a result of studies of the effects of several factors on the uniformity of flow. These included varying the width of the headers on the inlet and the outlet sides, using linearly and non-linearly tapered headers, and using channels of different widths. Out of all the factors studied, it has been found that the widths of the inlet and outlet side headers were the most important. The effects of using linearly tapered headers, multi-stepped headers and using the baffle plates and baffle tubes have also been studied. The effects of channel layouts on the flow and temperature uniformity inside a micro channel heat exchanger have also been studied. Several parallel channel layouts, serpentine layouts and distributor layouts have been compared based on average surface temperature, temperature non-uniformity and pumping power.
Thus, a heat exchanger for photovoltaic panels solving the aforementioned problems is desired.